EAS systems of the type described above, are, for example, disclosed and claimed in U.S. Pat. No. 3,665,449 (Elder et al.). As set forth at Col. 5, lines 10 to 39 therein, a dual status marker of the type described above may be desensitized, i.e., the high-coercive force section thereof magnetized, by placing the marker in the field of a large permanent magnet of sufficient intensity, and gradually removing the field, such as by withdrawing the marker therefrom. As also there disclosed, such a magnetization operation may be effected by imposing on the marker a unipolar pulsed field of gradually decreasing intensity.
While such techniques may be useful in many areas and with the markers affixed to a wide variety of articles, the magnetic fields associated therewith have been found to unacceptably interfere with magnetic states associated with certain articles. For example, the compact size and popularity of prerecorded magnetic audio and video cassettes make such articles frequent targets for shoplifters, and hence likely articles with which anti-theft markers would be used. At the same time however, such affixed markers would be desirably desensitized upon purchases, and it has been found that prior art desensitizing apparatus such as described above may unacceptably affect signals prerecorded on magnetic tapes within the cassettes.
To avoid such deleterious effects on prerecorded magnetically sensitive articles, it is also known to provide apparatus in which a steady-state field is produced which rapidly decreases in intensity with increased distance from the apparatus. Thus, such an apparatus improves the likelihood of magnetizing high-coercive force sections of a marker brought close thereto without interfering with the magnetic signals recorded on tapes within a cassette to which the marker is affixed. See U.S. Pat. No. 4,499,444 (Heltemes et al.). The apparatus described by Heltemes et al. comprises a permanent magnet assembly which includes at least one section of a permanent magnet ferromagnetic material having two substantially opposed major surfaces and a pair of pole pieces each of which is proximate to and extends over a major portion of the major surfaces and terminates proximate to the other pole piece, leaving a gap therebetween of substantially constant width extending along the length of the permanent magnet material. The permanent magnet material is substantially uniformly magnetized to present one magnetic polarity at one of the major surfaces and the opposite polarity on the other major surface. The pole pieces in turn concentrate external magnetic lines of flux resulting from the magnetized material near the gap. The resultant external magnetic field decreases rapidly with increasing distance from the gap, and enables a marker to be moved relative to the gap to magnetize the section of said high coercive force material within the marker while not altering magnetic states such as may exist within an article to which the marker is secured.
An apparatus such as described by Heltemes et al. has generally been found to be satisfactory so long as it is used with markers of a single type, and whose magnetizable components all have a coercive force within a given range, such that the field intensity at the working surface of the apparatus is controlled to appropriately magnetize those components while not adversely affecting magnetically sensitive articles. Conversely, it has been found that when the apparatus is used with markers nominally of the same type, but in which the value of the coercive force varies over a relatively wide range of allowed values, certain conditions may cause unsatisfactory results.
For example, to prevent adverse effects on magnetically sensitive articles with which the markers are desirably used, the field intensity at some distance from the working surface of the apparatus at which such magnetically sensitive articles are to be located, must be below certain design limits. However, a practical apparatus desirably has an effective operable range extending a short distance above the surface within which all allowed materials must become magnetized. Some materials having coercive forces near the highest allowed value and positioned near the outer edge of the allowed range, i.e., in the weakest fields, may not become sufficiently magnetized. And, since there is typically a reverse directed back field, which is particularly strong near the surface of the apparatus, such back fields may be sufficient to reduce the magnetization state in materials near the surface and having coercive forces near the lowest allowed value. Such reduced magnetization levels could, in turn, inadequately bias the low coercive, high permeability material of the marker, such that the response of the marker would be inadequately altered. Such effects are further compounded and totally unacceptable results may occur, if markers of significantly different types, each having magnetizable materials having coercive forces in significantly different ranges are used with the same apparatus.
Permanent magnet assemblies such as those described by Heltemes et al. are designed to concentrate magnetic flux across a gap defined by specially configured pole pieces. While most of the flux may flow across the gap, there may also be an appreciable fringe, or back field having an opposite polarity to that across the gap. Even at a relatively short distance above the gap, such as at the working surface of the apparatus described above, such a back field may have an intensity of several percent of the forward flux flowing across the gap. In constructions like that shown in the referenced patent, at short distances above the gap, the back field may exceed 6% of the field directly over the gap.
The desensitizable markers used in EAS systems may have magnetizable elements in a range of coercive forces. For example, the apparatus may be desirably designed to operate with three distinct types of markers, all having at least one responder section of a high permeability, low coercive force material such as permalloy and at least one magnetizable section. One such marker has a magnetizable element with a coercive force in the range of 24,000-28,000 A/m (300 to 350 oersteds), a second type has a magnetizable element with a coercive force in the range of 14,400-18,400 A/m (180 to 230 oersteds), and a third type has a magnetizable element with a coercive force in the range of 4,800-7,200 A/m (60-90 oersteds). Such markers may, for example, be type QT Quadratag.TM., Type WH-0117 Whispertape.TM. and type QTN Quadratag.TM. markers, respectively, all of which are sold by Minnesota Mining and Manufacturing Company (3M), St. Paul, Minn.
It has been generally observed that a field of about 1.5 times the coercive force is needed to reliably magnetize such magnetizable materials, while oppositely directed field intensities of about 0.5 times the coercive force may appreciably lower the residual magnetization. Thus, field intensities of about 1.5 times the coercive force are required to magnetize such elements at the maximum distance from the working surface at which a marker would reasonably be expected to be. Based on normal field attenuation, the field right at the working surface would be appreciably higher, e.g., about twice the coercive force. And, a back field 6% that of the primary field would then be about 12% of the coercive force. Thus, a forward field of sufficient intensity to magnetize elements having a maximum coercive force of about 28,000 A/m (350 oersteds) would have a back field of about 3360 A/m (42 oersteds). Such an oppositely directed back field could then adversely affect, e.g., partially demagnetize, a magnetizable element having a coercive force of less than 8000 A/m (100 oersteds).
The problem is accentuated when highly anisotropic magnetizable elements are used in markers. For example, such an anisotropic material, having a nominal coercive force of about 25,600 A/m (320 oersteds) is used in the type QT Quadratag.TM. markers discussed above. Since the alignment of the marker when used in the apparatus is uncontrolled, intensities of 48,000-64,000 A/m (600-800 oersteds) are necessary to reliably magnetize such materials. Such intensities at the working surface of the apparatus may correspond to an intensity of about 96,000 A/m (1200 oersteds). And such a front field could have associated back field of about 6400 A/m (80 oersteds), which is sufficient to adversely affect the magnetization of magnetizable elements having a coercive force less than about 14,400 A/m (180 oersteds), such as markers of the second and third types identified above.
U.S. Pat. No. 5,187,462 (Montean) addresses the back field problem by using a plurality of magnetic assemblies, each presenting a successively weaker field at the working surface, where each successively weaker forward field is sufficiently intense to restore the magnetization in an element partially demagnetized by the back field of a preceding assembly. However, even the use of the plurality of magnet assemblies taught by Montean does not totally eliminate the effects of back fields. Some back field always remains, and consequently, some markers may be accidently demagnetized.
Today, the retail recording industry is considering the option of applying markers at various locations on the article, and in various orientations. The Heltemes and Montean references presume that the location and orientation of the marker is known. Markers that are not properly oriented with respect to the direction of motion of the marker over the desensitizing apparatus may not be magnetized. Furthermore, multiple markers may be used, and these markers may be rotated with respect to each other. This increases the chance that one of the markers on the article will not be properly magnetized by the desensitizer.
It would be desirable to have a demagnetizing apparatus: (1) that eliminated the back field problem, (2) whose magnetic field strength decreased rapidly away from the magnet assembly, and (3) that functioned independently of the orientation of the marker with respect to the direction of travel of the marker over the apparatus.